Conventionally, as a surface coated cutting tool having a hard coating layer adhered to a surface of a substrate, tools in which a single or a multilayer of hard coating layers, such as a TiC layer, a TiN layer, a TiCN layer, an Al2O3 layer, or a TiAlN layer, are adhered to a surface of a hard substrate such as cemented carbide, cermet, or ceramics, have been used for various applications.
Recently, along with the requirement for higher efficiency of cutting process, further improvements in fracture resistance and wear resistance are required. Especially, cutting by which a cutting edge is subject to a large impact, for example, heavy interrupted cutting of metal, is increasing. The conventional cutting tools, however, have the following problem. That is, under the severe cutting conditions, the hard coating layer fails to endure the large impact, so that chipping or peeling of the hard coating layer are apt to occur, which may trigger accidental damage on the tool, such as the fracture of its cutting edge and the occurrence of abnormal wearing, making it impossible to improve the tool life.
With this in view, patent literature 1 describes that an aluminium oxide layer being dense and having high fracture resistance is attainable by optimizing the particle size and the layer thickness of an aluminum oxide layer, and setting the orientation coefficient (texture coefficient) on the (012) plane to 1.3 or more.
Patent literature 2 describes that setting the orientation coefficient (texture coefficient) on the (012) plane to 2.5 or more facilitates release of the residual stress of the aluminium oxide layer, enabling the fracture resistance of the aluminium oxide layer to be improved.
With the methods of improving entirely the fracture resistance of the aluminium oxide layer, namely improving the toughness of the aluminium oxide layer, as described in the above-mentioned patent literature 1 and patent literature 2, it is possible to prevent the tool damage on the rake face particularly apt to cause accidental fracture, chipping, and the like. In contrast, on the flank, a reduction in the hardness of the aluminium oxide layer facilitates to propagate abrasive wear due to the contact with a work material. As the result, there is a limit to the tool life.
On the other hand, patent literature 3 describes that the peak on the (110) plane is maximized in the X-ray peak of X-ray diffraction analysis of the aluminium oxide layer, in order to improve the adhesion of the aluminium oxide layer.
Patent literature 4 describes that the fracture resistance of the aluminium oxide layer can be improved by optimizing the peak of the (012) plane, the (104) plane, the (110) plane, and the (116) plane in the X-ray peak of X-ray diffraction analysis of the aluminium oxide layer.
With the methods of improving the adhesion of the aluminium oxide layer as described in the above-mentioned patent literature 3 and patent literature 4, the propagation of layer peeling and wear can be prevented. However, the adhesion of the aluminium oxide layer on the rake face is too high, and hence the underlying hard layer may flake early. Therefore, if a large impact is exerted, an accidental large fracture might be generated. Whereas on the flank, if a crack occurs in the aluminium oxide layer, a dissolved work material enters into the crack, facilitating adhesion of the work material. As the result, there is a limit to the tool life.    Patent literature 1: Japanese Patent No. 3325987    Patent literature 2: Japanese Unexamined Patent Publication No. 2003-025114    Patent literature 3: Japanese Unexamined Patent Publication No. 10-156606    Patent literature 4: Japanese Unexamined Patent Publication No. 2002-370105